In conventional passenger protection systems such as disclosed in U.S. Pat. No. 5,923,133 (JP 11-55984A), disconnection or shorting of electric wires in an airbag system is diagnosed by periodically supplying a small monitoring current to an airbag igniter circuit. For instance, as shown in FIG. 10, a squib 1 of an airbag is connected to a driving transistor TR1 and a switching transistor TR2 through respective resistors R1 and R2. The driving transistor TR1 is connected to an electric power source, and the switching transistor TR2 is grounded. The driving transistor TR1 is connected to a microcomputer 2 to be controlled thereby through an auxiliary transistor TR5. The switching transistor TR2 is also connected to the microcomputer 2 to be controlled thereby.
In this system, a small monitoring current Im much lower than the predetermined level is periodically or intermittently supplied to the squib 1 by slightly turning on the transistors TR1 and/or TR2 under control of the microcomputer 2, so that the squib 1 produces a small voltage. The microcomputer 2 diagnoses the airbag system by monitoring whether the small voltage is produced across the squib 1.
The monitoring current may be, as shown in FIG. 11, about 10 mA in amplitude and rises and falls at a change rate of about 2 mA/μs and −2 mA/μs. This current change generates electrical noise, which changes electric field strength sharply as shown in FIG. 12 and is received by a vehicle-mounted radio receiver. To remove the noise, as shown in FIG. 13, capacitors C1 and C2 are connected to both terminals of the squib 1 and ground to operate as noise filters. These capacitors C1 and C2 must be provided outside an electronic circuit board, resulting in increased cost.